The invention relates to a wheel lock-up prevention apparatus for use in the braking system of a motor vehicle. The apparatus includes one relay valve whose primary side is actuated by the pressure controlled by the driver-operated master cylinder and whose secondary circuit includes a valve member which controls the pressure in one or more wheel brake cylinders. The wheel brake cylinder is controlled by alternately establishing communication from the wheel brake cylinder to a source of pressure or to a relief orifice. The apparatus includes two valves connected ahead of the control chamber of the relay valve and one of these valves is the inlet valve establishing and breaking the connection of the control chamber with a main brake control line coming from the driver-operated master cylinder whereas the other valve establishes and breaks a connection between the control chamber and a relief location. Two electromagnets associated respectively with the two valves can be actuated by an electric controller.
In the known wheel lock-up prevention apparatus of the type described, the gradual increase of pressure after each pressure drop is obtained cyclically, usually with the aid of an electronic controller. The steepness of the pressure increase, which is based both on legal requirements for braking efficiency and also on the response time of the magnetic valve, is such that the pressure increase of each and every pressure jump is so high that a renewed excessive braking effort cannot be prevented.